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Characterization of a subset of the basic-helix-loop-helix-PAS superfamily that interacts with components of the
J B Hogenesch1, W K Chan, V H Jackiw
1Department of Molecular Pharmacology and Biological Chemistry, Northwestern University Medical School, Chicago, Illinois 60611, USA.
Abstract:
In an effort to better understand the mechanism of toxicity of 2,3,7, 8-tetrachlorodibenzo-p-dioxin, we employed an iterative search of human expressed sequence tags to identify novel basic-helix-loop-helix-PAS (bHLH-PAS) proteins that interact with either the Ah receptor (AHR) or the Ah receptor nuclear translocator (ARNT). We characterized five new "members of the PAS superfamily," or MOPs 1-5, that are similar in size and structural organization to the AHR and ARNT. MOPs 1-4 have N-terminal bHLH and PAS domains and C-terminal variable regions. MOP5 contained the characteristic PAS domain and a variable C terminus; it is possible that the cDNA contains a bHLH domain, but the entire open reading frame has yet to be completed. Coimmunoprecipitation studies, yeast two-hybrid analysis, and transient transfection experiments demonstrated that MOP1 and MOP2 dimerize with ARNT and that these complexes are transcriptionally active at defined DNA enhancer sequences in vivo. MOP3 was found to associate with the AHR in vitro but not in vivo. This observation, coupled with the fact that MOP3 formed tighter associations with the 90-kDa heat shock protein than the human AHR, suggests that MOP3 may be a conditionally active bHLH-PAS protein that requires activation by an unknown ligand. The expression profiles of the AHR, MOP1, and MOP2 mRNAs, coupled with the observation that they all share ARNT as a common dimeric partner, suggests that the cellular pathways mediated by MOP1 and MOP2 may influence or respond to the dioxin signaling pathway.
Insights
Researchers identified five new bHLH-PAS proteins (MOPs 1-5) that interact with dioxin signaling pathway components. MOP1 and MOP2 dimerize with ARNT, influencing cellular responses to dioxin toxicity.
Area of Science:
- Molecular Biology
- Toxicology
- Genetics
Background:
- The mechanism of 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity involves the aryl hydrocarbon receptor (AHR) and its nuclear translocator (ARNT).
- Understanding novel proteins interacting with AHR and ARNT is crucial for elucidating dioxin signaling pathways.
Purpose of the Study:
- To identify and characterize novel basic-helix-loop-helix-PAS (bHLH-PAS) proteins interacting with the AHR or ARNT.
- To investigate the functional interactions and potential roles of these novel proteins in cellular pathways.
Main Methods:
- Iterative search of human expressed sequence tags (ESTs).
- Coimmunoprecipitation studies, yeast two-hybrid analysis, and transient transfection experiments.
- Analysis of mRNA expression profiles.
Main Results:
- Five novel bHLH-PAS proteins, termed MOPs 1-5, were identified.
- MOP1 and MOP2 were shown to dimerize with ARNT, forming transcriptionally active complexes.
- MOP3 associated with AHR in vitro but not in vivo, suggesting conditional activity.
Conclusions:
- MOP1 and MOP2, through dimerization with ARNT, may influence or respond to dioxin signaling pathways.
- MOP3's conditional activity suggests a regulatory role in response to unknown ligands.
- These findings expand the known interactome of the AHR/ARNT complex and offer new targets for studying dioxin toxicity.